Re: Why do additional cores reduce performance?

[Paolo Bonzini <pbonzini@redhat.com>]

On 16/12/2014 00:40, Oleg Ovechko wrote:
> A. Host Windows, 6 cores (no HT, turbo boost off): 6:23 (+- 10 secs)
> B. Host Windows, 1 CPU core (other are turned off in BIOS): 7:13 (+-10 secs)
> C. Host 1 core, Guest Windows 1 core: 7:15 - same as B, no degradation
> D. Host 6 cores, Guest Windows 1 core: 7:57
> E. Host 6 cores, Guest Windows 4 cores: 8:17

What is your benchmark?

Windows sometimes has scalability problems due to the way it does
timing.  Try replacing "-cpu host" with "-no-hpet -cpu
host,hv_time,hv_vapic".

> 3. Also I am unsure about HT. When I specify "cores=2",

I suppose you mean "threads=2".

> is there any
> guaranty that whole core with both HT parts is passed to VM? Or it can be
> mix of two real cores with separate caches?

It will be a mix.  Do not specify HT in the guest, unless you have HT in
the host _and_ you are pinning the two threads of each guest core to the
two threads of a host core.

Paolo